Method of fractionation



Oct. 10, 1939. P, D. BARTON METHOD OF FRACTIONATION Filed March 29, 1938 Patented ocr. 1o, 1939 UNITED STATES PATENTOFFICE METHOD OF FRACTIONATION Application March 29 2 Claims.

The present invention relates to a fractionating process for fractionating mixtures of normally gaseous hydrocarbons, and specifically to a novell process for supplying liquid reflux to such a fractionating system.

The common method of furnishing reflux liquid to a fractionating zone is to withdraw an overhead vaporous fraction and to condense this fraction by indirect heat exchange with an extraneous cold fluid, generally water. This same method is generally used in furnishing reflux liquid in the fractionation of normally gaseous hydrocarbons. However, when handling such material, it is possible to obtain a large amount of refrigeration by expansion of the vapors of the normally gaseous hydrocarbons, but heretofore no effort has been made to utilize such available refrigeration for cooling and condensing a portion of the overhead vaporous fraction from a fractionating zone to furnish liquid reflux to the fractionating zone.

It is, therefore, an object of the present inventon to provide a method of supplying liquid reflux to a fractionating zone in which use is made of the refrigerative properties of the vapors Withdrawn from the zone.

For a better understanding of the invention, reference should be made to the accompanying drawing in which the single ligure is a diagrammatic now chart oi' apparatus adapted to carry out the process of this invention.

For the sake of simplicity of description the present invention will be described in connection with its use for separating puried propane from a mixture of propane and other normally gaseous hydrocarbons such as methane, ethane, isobutane and butane, which may also include some of the unsaturated normally gaseous hydrocarbons such as ethylene, propylene, isobutylene, and butylene.

Referring now to the drawing, it will be seen that the apparatus comprises a source of supply such as storage tank A, a pump B fed from tank A, for forcing the crude mixture through a heater C to an ethane tower D. A condenser E is provided to partially condense the overhead vapors from tower D and expansion coolers F further cool and condense these vapors before they pass` 1938, Serial No. 198,793

(Cl. (i2-175.5)

.dense the overhead vapors from tower I flowing to reflux accumulator N.

Assuming that it is desired to obtain puried propane, which may also include some propylene, from a crude mixture of normally gaseous hydrocarbons, such as the customary refinery propane which generally containsv roughly 70% propane and propylene, 20-25% 'butane, 5-7% isobutane, 4-6% ethane, and a fraction of a per cent of methane, this crude mixture may befed through inlet pipe I to raw feed storage tank A, Where it is maintained under sulficient pressure to keep the mixture liquid, for instance approximately 200 pounds pressure at 80 F. The crude mixture is Withdrawn from tank A through linc 2 by pump F and forced under approximately 500 pounds pressure through line 3 to steam heater C Where its temperaturemay be raised to approximately 160 F. From thence the crude mixture flows through line 4 to ethane tower D Where it is fractionatedunder approximately 500 pounds per square inch pressure. The ethane and methane in the mixture will be vaporized and pass overhead through line 5 to condenser E where the major portion of the ethane and methane is condensed. 'I'he mixed liquid and vapor from condenser E flows through expansion coolers F Where they are further cooled to approximately or F., and thence through line 1 to redux accumulator G. Liquid is Withdrawn from accumulator G by pump 8 and forced through line 9 to the top of the tower D for use as reflux in the tower. The uncondensed vapors in accumulator G are Withdrawn through line I0 and pass through expansion valve II, where the pressure on the same is reduced and thence through line I2 to expansion coolers F where they pass in indirect heat exchange relation with the mixed liquid and vapor from condenser E. The reduction of pressure on the vapors flowing through expansion valve II will cause them to expand in the coolers and exert a refrigerative effect on the mixed liquid and vapors flowing therethrough. The expanded vapors are then withdrawn from the system through line I3. Then liquid bottoms frorntower D are withdrawn'through line I4 and circulated through reboiler H, wherein they are heated to approximately 200 F. at 500 pounds pressure. A portion of the bottoms are re-introduced into tower D through line I5, so that any ethane or methane remaining therein will be vaporized.

The remainder of the liquid bottoms pass through line I6 having expansion valve I1 therein to the propane tower I, the pressure on the liquid being reduced from 500 pounds per square inch to approximately 250 pounds per square inch in passing the expansion valve I1. The material introduced into tower I is fractionated therein under a pressure of approximately 250 pounds per square inch, the propane and propylene being vaporized and withdrawn therefrom as an overhead vapor fraction through line i9 to the cooler and condenser and M respectively wherein the vapors are cooled and partially condensed. The mixed vapor and liquid from the coolers and con densers L and M flows to the reflux accumulator N from whichy liquid is withdrawn through line 22 by pump 23 and forced to the top of tower I for use therein as reflux. The uncondensed vapors from accumulator L are removed from the system through valved line 24.

It is to be understood of course that the ex pansion coolers similar to those used for cooling and condensing the overhead vapor fraction from towerD likewise may be used for cooling and condensing the overhead vapor fraction from the tower I. For example, the cooling fluid used in condenser M may by expanded propane vapors be withdrawn from reux accumulator N. If such is the case the valve 25 in line 24 is closed and expansion 21 in valve line 26 is opened thereby by passing the vapors from line 24 through line 26, wherein they are permitted to expand and cool on passing expansion valve 21, to condenser M, where they pass in indirect heat exchange with the vapors being condensed therein.

From the condenser M the expanded vapors flow through valved line 28 returning to line 24 beyond the valve 25 therein and being removed from the system. Since the propane is frequently sold or used in a liquid form it is not always economically advisable to make use ofV the refrigeration obtainable by expanding it to condense reflux for the fractionation, but in case the propane is to be used in a gaseous state all or a portion of its vapors may be expanded and used to condense retlux as just described, when economically advisable to do so.

The bottoms from tower I which consist mainly of butane and iso-butane are withdrawn through line 28 to re-boiler J, where they are heated to approximately 215 F. at 250 pounds pressure, a portion of the heated liquefied bottoms being returned to the tower I through line 29 for removal of any remaining propane therefrom and the remainder owing through line 30 into cooler K and thence being removedfrom the system through line 3i.

What I claim and desire to protect by Letters Patent is:

1. A process for separating normally gaseous hydrocarbon` mixtures which comprises subjecting the mixture to reflux fractionation, subjecting the separated overhead ,gaseous light fractional product of the fractionation to cooling and partial condensation under a substantial superatmospheric pressure, separating the condensate from the uncondensed gas, releasing the pressure on such uncondensed gas to further cool it, utilizing such cooled uncondensed gas as a cooling medium in the specified cooling and partial condensing step, and utilizing said separated condensate as reflux in the specied fractionation operation.

2. A process for fractionating mixtures of normally gaseous hydrocarbons which comprises introducing the mixture to be filtered into a fractionating zone, therein separating said mixture into an overhead vaporous fraction and a liquid residue, withdrawing said vaporous and said residual fractions from said zone, flowing said vaporous fraction while under a substantial superatmospheric pressure in indirect heat exchange relation with a cold expanded hydrocarbon gas to partially condense said overhead fraction, separating the vaporous portion thereof from the liquid portion, returning said liquid portion to said fractionating zone as reflux, reducing the pressure on said vaporous portion to expand and cool the same, and utilizing the thus cooled vapors as the cold expanded hydrocarbon gas in the heat exchange step specified.

PAUL D. BARTON. 

